Roughness Factor-Dependent Transport Characteristic of Shale Gas through Amorphous Kerogen Nanopores

Abstract

In the past decades, shale gas has been recognized as the promising unconventional resource for global energy storage, and a clear understanding of the gas-transport characteristic within nonporous shale organic matter (i.e., kerogen) is fundamental for the effective development of shale reservoirs. In this regard, previous studies were generally conducted based on the ideally smooth nanochannels (e.g., graphite slit or tube) without considering the atomistic-scale roughness of the walls. Herein, using molecular dynamics (MD) simulations, we perform a systematical investigation on the gas-transport characteristic through amorphous organic nanopores constructed by realistic kerogen molecules. The results show that the gas-transport velocity in amorphous organic nanopores drops dramatically (40, 70, and 90%) only with tiny roughness factors (0.3, 0.6, and 1.2%) when compared with ideally smooth nanochannels. Further analysis of the potential energy surface and the particle trajectory justifies the entirely different gas-transport mechanisms in ideally smooth (surface diffusion) and relative rough (viscosity diffusion) organic nanopores. Besides, based on the insights of numerous MD simulations (pore sizes: 3−9 nm and system pressures: 5−50 MPa), a new analytical model that is able to consider the key effect of roughness factor on gas transport in organic-rich shale is developed, which is well verified with the experimental results. It is particularly found that the gas-transport capacity in organic-rich shale (∼1 nm of slippage length) would be enormously overrated as much as 2 orders of magnitude by the traditional cognition based on ideally smooth nanopores (∼100 nm of slippage length).

A Molecular Dynamics Investigation on Methane Flow and Water Droplets Sliding in Organic Shale Pores with Nano-structured Roughness

“…Previous MD-based research includes nanoscopic simulations of fluid flow, droplet wetting and droplet interactions with smooth and rough solid surfaces. The effect of roughness on the behavior of water nanodroplets (Yaghoubi and Foroutan 2018), wetting transition (Khan and Singh 2014;Koishi et al 2009) and gas flow characteristics (He et al 2019;Yu et al 2020) have been reported. Fang et al (2019) performed MD simulation to elaborate on the mechanisms of extracting hydrocarbon using CO 2 in a nanoscale groove constructed by silicon.…”

“…However, it is very challenging to measure the shale roughness whose length scale is less than 5 nm. However, molecular dynamics simulations have been used extensively as appropriate tools to understand the fundamentals of rock-fluid interaction in organic shale pore, for example (He et al 2019;Yu et al 2020) have studied the roughness (length scale is at 2 nanometers) effect on shale gas transport. Besides, structured surface (with close length scale in our simulation) has also widely been applied in related roughness studies via MD (Khan and Singh 2014;Koishi et al 2009;Yaghoubi and Foroutan 2018).…”

Effect of Surface Type on the Flow Characteristics in Shale Nanopores

“…Such different slip behavior of methane molecules on the quartz and graphene surface is interesting; the different liquid-solid interactions are generally regarded as the main reason, which is mainly dominated by the atom's parameters σ and ε [64,65] in Table 2 for the CH 4 molecules confined in various nanopores. In addition, the breakdown of gas molecules slippage on the quartz surface is especially owing to the inherent atomic roughness of the surface by Yu et al [43,66], especially, the potential energy roughness of the quartz surface is higher than that on graphene surface up to two magnitude of orders. Thus, the gas molecules collide with the quartz surface and just go round and round and not move along the walls as what the molecules act on the graphene surface [40].…”

“…The roughness in atomic-scale roughness plays a nonnegligible role in interrupting the molecules moving along the pore wall. Furthermore, Yu et al also found that the roughness has great effect on the mass-transport of shale gas using a series of rough kerogen structures [43]. While in the MD simulation, it is not easy to control the roughness of the kerogen structure [43].…”

Reduced methane recovery at high pressure due to methane trapping in shale nanopores

“…While shale nanopores are both rough and irregular, smooth graphite slit pore models have been widely used as a simplified corollary for kerogen nanopores in shale [70][71][72] . Surface roughness affects both the flow dynamics and the adsorption capacity of methane in pores [73][74][75][76] . However, given that our primary goal is to qualitatively compare retention in our model with neutron scattering observations, it warrants the use of smooth surface.…”